Wiring help, trying to understand diagram

[Adarian]

Bus won't crank from the front of rear controls. It will start if I jump the starter solenoid.
Wire 75a at the starter solenoid, does that provide power or ground to the starter solenoid?
Wire 75a goes to the neutral start relay.What is the starter lock out job?
http://www.palofitness.com/bus/wiringa.jpg wiring diagram

Black wires are always live when master switch is turned on.
White wires are the main ground wires.

[Cary and Don]

You may already know this, but the starter will not crank unless you are in neutral.  The lock out may also stop it from starting under other conditions for you bus. Something we frequently forget when trying to start the bus.

Don and Cary
GMC4107
Neoplan AN340

[fe2_o3]

On the transit coaches we had at work The starter wouldn't turn over in front unless the trans was in neutral, the maxi was set and some one was in the seat. The rear didn't care if the seat was occupied....Cable

[zubzub]

while you're at it, some transits won't move if the doors are ajar. and who knows what else.....lot's of "safety" stuff.

[Len Silva]

Looking at your schematic, it looks like the the neutral start relay must be operated and the starter lock out not operated in order to apply a ground to term 75A of the start solenoid.  The starter lockout may be operated by a fuel pressure switch to prevent the starter from operating when the engine is running.

It looks like grounding term 75A on the starter solenoid would bypass all the safeties in the ground path.  There may well be other switches/relays in the battery path from term 16A, but I can't tell from the schematic.

[Adarian]

Looking at your schematic, it looks like the the neutral start relay must be operated and the starter lock out not operated in order to apply a ground to term 75A of the start solenoid.  The starter lockout may be operated by a fuel pressure switch to prevent the starter from operating when the engine is running.

It looks like grounding term 75A on the starter solenoid would bypass all the safeties in the ground path.  There may well be other switches/relays in the battery path from term 16A, but I can't tell from the schematic.

That is what I thought.
I have some voltage at 75a

The 10x wire at the starter lock out relay goes to the junction bar above, post number 4, it gets a feed from 10a which comes from the generator. That makes sense that if motor is running then generator would be charging and locks out the starter. The 10b wire at post 4 I have no idea what it is attached to it goes to the front of the bus.

[Len Silva]

You cannot tell for sure that you have 12 volts on wire 75A without disconnecting the wire from the solenoid.  You could be seeing back feed from term 16A.  Disconnect and check for voltage on the wire, should be zero or ground. 

[Adarian]

You cannot tell for sure that you have 12 volts on wire 75A without disconnecting the wire from the solenoid.  You could be seeing back feed from term 16A.  Disconnect and check for voltage on the wire, should be zero or ground. 

16a doesn't have voltage until I push the start switch.
I will still disconnect 75a and check to make sure.

[Adarian]

Got this all worked out. All works as it should.
The control relay is for the transmission gear selector.
If any gear is selected this relay is activated.
If neutral is selected relay is not activated.
This allows power to go to the neutral start relay, turning it on.
Allowing for wire 75a to be grounded through 75b.

Now that I have the starting circuit  figured out, on to the interlocks.

[Adarian]

Problem solved.
1. Charged batteries.
2. Replaced solenoid for the front, connected to master switch.
3. Before trying to start after first attempt wait 30 seconds. Even though bus has air starter there still is a time delay between starting attempts.

bus starts as should from front or rear.
All lights and electronics work.
just need to replace alternator belt and tension pulley.